The invention relates to a coupling for controllably transmitting torque, and to a method of controlling said coupling. The coupling comprises a first coupling member and a second coupling memberxe2x80x94especially a shaft and a coupling housingxe2x80x94which are supported so as to be rotatable relative to one another; the coupling comprises a friction coupling and a viscous coupling switched in parallel; the friction coupling comprises two sets of friction plates, the viscous coupling comprises two sets of VC plates respectively associated with one of two VC plate carriers. Couplings of the type described below are largely used as locks in differential drives of motor vehicles or as locks for connecting an additional drive axle in vehicle drives.
From DE 38 15 225 C2, there is known a differential drive with a friction coupling for which there has been proposed a particularly suitable, controllable variable speed drive.
A coupling which also serves to lock a differential drive and comprises a friction coupling and a viscous coupling switched in parallel is known from DE 34 26 460 C1, with said coupling being controlled automatically.
A further coupling of the above-mentioned type is known from DE-OS 22 09 879. However, in this case, the viscous coupling is not provided in the form of a multi-plate coupling.
Further couplings used for the above applications are described in EP 0 314 420, WO 86/02133 A1, U.S. Pat. Nos. 4,031,780, 4,058,027 and finally JP 1-72415 U and JP 63-312235 A.
It is an object of the present invention to provide a coupling of said type which can be controlled particularly well, and a method of controlling said coupling. The objective is achieved by providing a method of controlling a coupling for controllably transmitting torque. The coupling comprises a first coupling member and a second coupling memberxe2x80x94especially a shaft and a coupling housingxe2x80x94which are supported so as to be rotatable relative to one another. The coupling comprises a friction coupling and a viscous coupling switched in parallel. The friction coupling comprises two sets of friction plates and the viscous coupling comprises two sets of VC plates respectively associated with one of two VC plate carriers. The method is characterized in that for disengaging the coupling, the sets of friction plates of the friction coupling are axially separated from one another and that one of the VC plate carriers of the viscous coupling is completely disconnected from one of the coupling members. In particular, it is proposed that for an increasing engagement of the coupling, first the one of the VC plate carriers of the viscous coupling is coupled with one of the coupling members in a friction locking way, whereas the other one of the VC plate carriers of the viscous coupling remains permanently connected in a rotationally fast way to the other one of the coupling members. Thereafter, the friction coupling is engaged, with the sets of friction plates of the friction coupling remaining permanently connected in a rotationally fast way with one of the two coupling members.
In another aspect, a coupling for controllably transmitting torque, having a first coupling member and a second coupling memberxe2x80x94especially a shaft and a coupling housingxe2x80x94which are supported so as to be rotatable relative to one another is provided. The coupling comprises a friction coupling and a viscous coupling switched in parallel. The friction coupling comprises two sets of friction plates and the viscous coupling comprises two sets of VC plates each associated with one of two VC plate carriers. The sets of friction plates of the friction coupling are respectively connected in a rotationally fast way to the first and the second coupling member and axially loadable by an adjusting device. A first VC plate carrier of the viscous coupling is permanently connected in a rotationally fast way to the first one of the coupling members, and a second VC plate carrier of the viscous coupling is disconnectably connected to the second coupling member. The device also includes an adjusting device for axial adjustment connecting and disconnecting the second VC plate carrier of the viscous coupling in a friction locking way and closing and releasing the sets of friction plates of the friction coupling.
The inventive coupling combines the high torque transmitting capacity of a friction coupling with the extremely sensitive reaction behavior of a viscous coupling. In addition, in spite of the speed differential between the coupling members, the coupling has the characteristic of being completely separatable by disconnecting the viscous coupling.
The viscous coupling needs to transmit a relatively low torque only and does not require a hump function. The control behavior is improved as a result of the delay-free and self-controlling function of the viscous coupling.
To the extent that the coupling housing of the viscous coupling is used in some of the following embodiments in the function of a coupling plate, the housing has to be correspondingly robust.
According to an alternative, it is possible for a plate carrier to be connected to the coupling housing, with friction plates on said plate carrier, together with friction plates held in the second coupling member, forming a separate auxiliary friction coupling for connecting the viscous coupling housing.
It is proposed that preferred axial adjusting devices are those initially mentioned in DE 38 15 225 C2. These can optionally be provided with two or three ramp discs. In an embodiment with two ramp discs, one of the discs is held axially and the other one of the discs is axially spread apart if the two discs rotate relative to one another. In the embodiment with three ramp discs, the middle one can be held axially and if the discs rotate relative to one another and especially if the middle disc rotates, the two outer ramp discs, especially via different ranges of rotational angles, can be spread apart from said middle disc one after the other.
The ramp discs are preferably supported relative to one another by three balls held in pairs of grooves.
The rotatable ramp discs are adjusted by an electric motor which, when stopping, is braked by a spring-loaded brake plate package. When the motor is switched on for adjusting purposes, a magnet first releases the brake plate package.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims and upon reference to the accompanying drawings.